Several independent risk factors have been associated with cardiovascular disease. These include hypertension, increased fibrinogen levels, high levels of triglycerides, elevated low density lipoprotein (LDL) cholesterol, elevated total cholesterol, and low levels of high density lipoprotein (HDL) cholesterol. HMG CoA reductase inhibitors (e.g., statins) are useful for treating conditions characterized by high LDL cholesterol levels. It has been shown that lowering LDL cholesterol is not sufficient for reducing the risk of cardiovascular disease in some patients, particularly those with normal LDL cholesterol levels. This population pool is identified by the independent risk factor of low HDL cholesterol. The increased risk of cardiovascular disease associated with low HDL cholesterol levels has not yet been successfully addressed by drug therapy (i.e., currently there are no drugs on the market that are useful for raising HDL cholesterol). See, e.g., Bisgaier et al. (1998) Curr. Pharm. Des. 4:53-70.
Targets for the development of therapeutic agents for cardiovascular disease, diseases associated with cardiovascular disease, such as syndrome X (including metabolic syndrome), and other pathologies such as, diabetes, obesity and cancer include transcription factors involved in regulating lipid metabolism and homeostasis.
The peroxisome proliferator-activated receptors (PPARs) are transducer proteins belonging to the steroid/thyroid/retinoid receptor superfamily. The PPARs were originally identified as orphan receptors, without known ligands, but were named for their ability to mediate the pleiotropic effects of fatty acid peroxisome proliferators. Three mammalian PPARs have been isolated: PPARγ, PPARα and PPARδ (PPARβ, NUC1). These receptors function as ligand-regulated transcription factors that control the expression of target genes by binding to their responsive DNA sequence as heterodimers with RXR. The target genes encode enzymes involved in lipid metabolism and differentiation of adipocytes.
PPARγ has been shown to be expressed in an adipose tissue-specific manner. Its expression is induced early during the course of differentiation of several preadipocyte cell lines. Additional research has now demonstrated that PPARγ plays a pivotal role in the adipogenic signaling cascade. PPARγ also regulates the ob/leptin gene which is involved in regulating energy homeostasis and adipocyte differentiation, which has been shown to be a critical step to be targeted for anti-obesity and diabetic conditions.
In an effort to understand the role of PPARγ in adipocyte differentiation, several investigators have focused on the identification of PPARγ activators. One class of compounds, the thiazolidinediones, which were known to have adipogenic effects on preadipocyte and mesenchymal stem cells in vitro, and antidiabetic effects in animal models of non-insulin-dependent diabetes mellitus (NIDDM), were also demonstrated to be PPARγ-selective ligands (Lehmann et al. (1995) J. Biol. Chem. 270:12953-12956). More recently, compounds that selectively activate murine PPARγ were shown to possess in vivo anti-diabetic activity in mice.
Activators of PPARγ, such as troglitazone, have been shown in the clinic to enhance insulin action, reduce serum glucose and have small but significant effects on reducing serum triglyceride levels in patients with NIDDM diabetes. See, for example, Kelly et al. (1998) Curr. Opin. Endocrinol. Diabetes 5(2):90-96, Johnson et al. (1997) Ann. Pharmacother. 32(3):337-348 and Leutenegger et al. (1997) Curr. Ther. Res. 58(7):403-416. The mechanism for this triglyceride lowering effect appears to be predominantly increased clearance of very low density lipoproteins (VLDL) through induction of lipoprotein lipase (LPL) gene expression. See, for example, B. Staels et al. (1997) Arterioscler. Thromb. Vasc. Biol. 17(9):1756-1764.
Fibrates are a class of drugs which may lower serum triglycerides by 20-50%, lower LDL cholesterol by 10-15%, shift the LDL particle size from the more atherogenic small dense to normal dense LDL, and increase HDL cholesterol by 10-15%. Experimental evidence indicates that the effects of fibrates on serum lipids are mediated through activation of PPARα. See, for example, Staels et al. (1997) Pharm. Des. 3(1): 1-14. Activation of PPARα results in transcription of enzymes that increase fatty acid catabolism and decrease de novo fatty acid synthesis in the liver resulting in decreased triglyceride synthesis and VLDL production/secretion. In addition, PPARα activation decreases production of apoC-III. Reduction in apoC-III, an inhibitor of LPL activity, increases clearance of VLDL. See, for example, Auwerx et al. (1996) Atherosclerosis, (Shannon, Irel.)124(Suppl.):S29-S37.
Evidence suggests that PPARδ also controls the peroxisomal beta-oxidation pathway of fatty acids. Activators of PPARδ have been shown to promote reverse cholesterol transport, which can raise HDL cholesterol levels. See, Oliver et al. (2001) Proc. Natl. Acad. Sci. USA 98(9):5306-5311. It has also been shown that PPARδ activators inhibit the formation of the inflammatory mediator's inducible nitric oxide synthase (iNOS) and tumor necrosis factor (TNF). See, International Publication No. WO 02/28434 to Buchan et al. Moreover, it has been shown that PPARδ, unlike PPARγ or PPARα, represents a β-catenin/Tcf-4 target with particular importance for chemoprevention (He et al. (1999) Cell 99:335-345).
The identification of compounds which modulate PPARδ provides an opportunity to probe PPARδ-mediated processes and discover new therapeutic agents for conditions and diseases associated therewith, such as cardiovascular disease, atherosclerosis, diabetes, obesity, syndrome X and malignant diseases.